5-Azacytidine, a cytidine analog, is capable of causing undermethylation of newly synthesized DNA, and can reactive dormant genes in tissue culture cells and in baboons. This drug has been administered to several patients with Beta thalassemia and sickle cell anemia; Gamma globin synthesis was selectively increased in the bone marrow cells of each patient after treatment. The mechanism of augmented fetal hemoglobin production has been under intense investigation during the past year. A study of hemoglobin F production in erythroid progenitor and precursor cells of bone marrow suggests that 5-azacytidine directly alters the globin biosynthetic program in these cells. In addition, study of Gamma globin gene expression in a hybrid mouse erythroleukemia cells line containing the human 11 chromosome also indicates that the Gamma globin genes are selectively reactivated by 5-azacytidine in this cell line. These results indicate that 5-azacytidine probably does act, at least in part, by hypomethylating critical control sequences around specific genes. We have also studied the usefulness of repeated courses of 5-azacytidine in patients with Beta thalassemia. In two patients, chronic administration of the drug did indeed result in an increased Gamma globin synthesis, but a decline in Beta globin synthesis and presumed toxicity to the erythron blunted the clinical response. 5-Azacytidine was not capable of eliminating the transfusion requirement in these patients. Further studies with this drug have been postponed pending further studies of the effect of other cytotoxic agents on fetal hemoglobin production in humans.